A lead-acid storage battery is a device which converts chemical energy into electrical energy by means of chemical reactions taking place between a liquid electrolyte (consisting of an aqueous solution of sulfuric acid, H.sub.2 SO.sub.4), and the active materials of a plurality of lead-based positive and negative plates. These active materials are lead dioxide (PbO.sub.2) for the positive plates and sponge lead (Pb) for the negative plates.
A conventional lead-acid battery assembly consists of the cell elements, intercell connectors, a container, a cover, vent plugs and terminal posts. A cell element is formed by stacking together alternate negative and positive plates, with separators. The lugs of the negative plates in the cell element are joined together by a cast-on-strap joint. The same is done to the lugs of the positive plates in the cell element. The cast-on-strap joint of each group of plates is used to connect them in series with the plate groups of adjacent cells, in such a way that the voltage of the battery equals the sum of the voltages of the individual cells. The voltage of a single cell is nominally 2 volts, regardless of the size or quantity of plates. Therefore, a 12-volt battery has six cells and a 6-volt battery has three cells.
The steps followed to manufacture a lead-acid battery are: plate grid casting and preparation of active material pastes; paste application to the plate grids; paste curing; cell element integration operations; plate lugs joining by cast-on-strap process; subsequent battery assembly processes; electrolyte addition to battery cells; electric formation of the battery and final assembly and testing.
The present invention relates to the cast-on-strap process used to join the negative and the positive plates in the cell elements of lead-acid batteries. A casting machine commonly employed to manufacture the cast-on-strap joints comprises four working stations. At station 1, the plate groups are loaded into the machine, their lugs are turn downward and the plates are vibrated to align them up. Then, the plates are moved to station 2, where a two-step cleaning treatment is given to the surface of the plate lugs. The first step of the cleaning treatment involves brushing by means of a rotating horizontal steel brush in order to eliminate lead oxide skins and active material pastes. The second step of the cleaning treatment involves the application, at room temperature, of a film of liquid flux to the freshly brushed plate lugs surface by means of a rotating felt brush or by means of a flux-impregnated sponge. Subsequent processing at station 3 of the machine involves the manufacturing of a pair of lead-based cast-on-strap joints for each cell element, using a suitable water-cooled and preheated metallic mold. In order to do this, the mold cavities are filled, either by gravity or pressurized injection, with a lead-based molten strap alloy of a suitable chemical composition. Then, the flux-impregnated plate lugs are immersed into the molten strap alloy. The cast-on-strap joints are formed by subsequent solidification of the molten strap alloy in the strap mold. The final processing step, carried out at station 4, involves the withdrawal of the cell elements, with the cast-on-strap joints already formed, from the machine.
A characteristic feature of the cell elements, produced by the conventional cast-on-strap process just described, is the presence of a substantial amount of porosity located mainly at the interface between the plate lugs surface and the cast-on-strap joints. This porosity is easily visible in cross-sectioned cast-on-strap joints. This type of defect may result in an accelerated corrosion at the pores, with a consequent reduction in the useful life of the batteries.
In order to determine the causes of the formation of porosity in the cast-on-strap joints obtained by employing the conventional cast-on-strap process, an extensive research program was carried out. The research led to the development of the present invention. It was discovered that the pores found inside the cast-on-strap joints are due mainly to the presence on the plate lugs surface of residues of lead oxides, greases, oils, active material pastes, fluxes, residual reaction products and humidity, all of which produce substantial volumes of gas on heating in contact with the molten lead-based strap alloy. The gases generated during solidification of the cast-on-strap joints cannot be completely eliminated, especially under the fast cooling conditions existing in the metallic mold. Thus, a considerable amount of gas bubbles remain trapped inside the solidified cast-on-strap joints obtained. It was concluded that pore-free cast-on-strap joints cannot be reproducibly produced by employing a conventional cast-on-strap machine. This makes it mandatory to develop a device truly capable of producing pore-free cast-on-strap joints.
The present invention relates to an apparatus for cleaning and chemical activation of a plurality of plate lug surfaces. When this apparatus is used, coupled to a conventional cast-on-strap machine, produces pore-free cast-on-strap joints for lead-acid batteries. The apparatus is based on a process described in a previous patent application (Ser. No. 08/941,531, filed on Sep. 30, 1997). This process comprises the steps of cleaning a plurality of positive and negative plate lugs by a combined action of a liquid flux and ultrasonic vibration, water rinsing the plate lugs under ultrasonic vibration, and drying the plate lugs by gas blowing. The gas blowing occurs prior to the formation of the cast-on-strap joints. The cast-on-strap joints are formed, following the conventional cast-on-strap process, by introducing the plate lugs into a molten lead-based strap alloy contained in a strap mold and solidifying the molten strap alloy in the strap mold.
The use of the apparatus has the present invention replaces the steps of brushing the plate lugs employing a rotating steel brush and application of a film of liquid flux to the plate lugs by means of a rotating felt brush or by means of a flux-impregnated sponge. These operations are commonly carried out at the second station of a conventional cast-on-strap machine. It is an object of the present invention to provide an apparatus for the manufacture of pore-free cast-on-strap joints.
This and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.